1. Field of the Invention
The present invention relates to a polishing apparatus for polishing semi-conductor wafers evenly and in mirror-surface, and in particular to a polishing apparatus enabling to control polishing amount around peripheral parts of the semi-conductor wafer.
2. Description of the Related Art
In general, a semi-conductor wafer is subjected to a so-called mirror-face polishing process for providing a mirror face with flatness of high precision and without strain. For performing the mirror-face polishing process, generally plural sheets of the semi-conductor wafer are polished simultaneously, so-called batch type polishing apparatus is served. With respect to the batch type polishing apparatus, explanation will be made in reference to FIGS. 10 and 11 of the attached drawings.
As shown, this polishing apparatus is equipped with a rotary shaft 1a, a polishing level block 1 provided on the upper end of the rotary shaft 1a, a polishing cloth pasted on the upper surface of the polishing level block 1, a polishing head 3 disposed rotatably and movably vertically at a position eccentric from the rotary shaft 1a, a plurality of plate holders 6 provided on the outer periphery of the polishing head 3, and a polishing plate 4 attached to the lower surface of the polishing head 3 and to be positioned by the plate holder 6.
The semi-conductor wafer 5 are, as shown in FIG. 11, arranged in the circumferential direction of the polishing plate 4, and the plural sheets are pasted to the surface of the polishing plate 4.
For polishing the semi-conductor wafer 5 by means of this polishing apparatus, the polishing level block 1 is rotated in an arrow direction around the rotary shaft 3a, and at the same time the polishing head 3 at the eccentric position with respect to the polishing level block 1 is rotated in the arrow direction. Subsequently, as dropping a polishing liquid A from a center of the polishing cloth 2 (an upper part of the rotary shaft 1a), the polishing head 3 is moved down to cause the semi-conductor wafer 5 to press to the polishing cloth 2.
In this conventional polishing apparatus, for polishing, the polishing liquid A is, as seen in FIG. 10, poured to the center of the polishing cloth 2 and flows in the outer circumference of the polishing cloth 2. As a result, the polishing liquid is much supplied to the circumference of the polishing 5, so that the polishing is progressed more in the circumference of the semiconductor wafer 5 than the central part (inside part), so-called excessive polishing (sags) takes place.
With respect to sags caused at the circumferential part of the semiconductor wafer, JP-A-11-114806 proposes that there is furnished a polishing face adjusting ring encircling the outer circumference of one sheet of wafer, so that the pressing force of the polishing face adjusting ring is controlled by fluid pressure so as to exert uniform polishing force to the semi-conductor wafer for uniformly polishing the semi-conductor wafer.
In the conventional polishing apparatus using the polishing face adjusting ring, the polishing face adjusting ring is pressed to the polishing cloth by fluid pressure and at the same time the pressure is controlled. As to the control of pressure by the fluid pressure, if the structure of the polishing apparatus is complicated, it is difficult to adjust pressure by the fluid pressure, and the pressure of the polishing face adjusting ring is varied as a time passes or depending on positions, and it is difficult to always effect pressure all over the ranges. In particular, in the batch type polishing apparatus where plural sheets of semi-conductor wafers are pasted on one sheet of plate for performing the polishings concurrently, the polishing face adjusting ring encircling the semi-conductor wafer is of large-diameter, and it is difficult to adjust all over pressing face of the polishing face adjusting ring and maintain the uniform force.
Even if the pressure is controlled by the polishing face adjusting ring to exert the uniform polishing force on the semi-conductor wafers, occurrences of sags could not be prevented and a substantial solution has not yet been realized.
Further, a semi-conductor wafer is made by producing a semi-conductor ingot of single crystal through, for example, a Czochralski method from polycrystal, slicing the ingot into predetermine thickness by means of such as a multi-wire sawyer, grinding the sliced semi-conductor wafer by a grinding apparatus, and further mirror-polishing it by a polishing apparatus.
But recently, accompanied with high integration of semi-conductor device, a demand has been arisen in the semi- conductor wafer as to the degree of flatness, and in the mirror-polishing process, a problem occurs about excessive polishing (outer circumferential sags) in the outer peripheral part of the semi-conductor wafer. It is assumed that one cause is owing to influences by sinking or depressing the abrasive cloth, and measures therefor have been practiced for moderating depression of the polishing cloth into the semi-conductor wafer by hardening the polishing cloth itself or previously pushing the polishing cloth.
For providing the polishing effect by the retainer ring, one of important factors is a distance (clearance) between the semi-conductor wafer and the retainer ring.
Namely, showing in FIGS. 21 and 22 as the results of simulating pressure dispersions of the retainer ring, the semi-conductor wafer and the polishing cloth by a static analysis, when the clearance C is large as shown in FIG. 21, since the polishing cloth 211 once crushed by the retainer ring 212 recovers until contacting the semi-conductor wafer 213 by compression elastic modulus of the polishing cloth 211, large change does not occur in pressure of the peripheral part 213c of the semi-conductor wafer 213, and reaction force of the peripheral part 213c receiving from the polishing cloth 211 becomes large, and an effect for furnishing the retainer ring 212 is not available. When the clearance C is small as shown in FIG. 22, the reaction force of the peripheral part 213c receiving from the polishing cloth 211 by the retainer ring 212 becomes small, and the effect for furnishing the retainer ring 212 can be sufficiently obtained.
Accordingly, if bringing the clearance to 0 unlimitedly, it is possible to obtain the retainer effect at the maximum, but actually it is difficult to process an inner diameter of the retainer ring equal to the diameter of the semiconductor wafer, taking dispersion of the diameter of the semi-conductor wafer and dispersion of the inner diameter of the retainer ring into consideration.
A method of checking face-sags in the wafer using the retainer ring is disclosed in JP-A-5-326468, and the disclosed polishing method carries out the polishing under a condition where the circumference of the wafer is attached with a ring of thickness smaller than a finished thickness of the wafer and checking the face sags.
But this disclosed method cannot bring the clearance to 0 unlimitedly, because the clearance is fixed during polish processing, and the retainer effect cannot be exhibited at the maximum.
Therefore, such a wafer polishing method has been demanded which can display the effect of the retainer ring to the most and prevent sags in the peripheral part of the wafer.
The invention has been realized to solve such problems, and it is an object of the invention to provide a polishing apparatus which polishes the semi-conductor wafers under a condition before recovering the polishing cloth (a deformed state), thereby to mirror-finish the semi-conductor wafers of flatness of high precision. It is another object of the invention to provide a polishing apparatus which prevents any invasion of excessive polishing liquid, easily controls a pressing force, has no probability of varying the pressing force, and is ready for being applied to such a polishing apparatus where a plurality of semi-conductor wafers are pasted on one sheet of plate for performing simultaneous polishings.
First, Inventors noticed properties of the polishing cloth for solving the problems.
Namely, when pressure to the polishing cloth is released, the polishing cloth gradually recovers an original state. Showing the relationship between pressure and displacing in FIG. 12, as apparently from this view, deformation of the polishing cloth 2 does not instantly recover the original state if removing pressure (load), but slowly recovers.
As is seen, the polishing cloth 2 is visco-elastic, and if it is pressed by the semi-conductor wafer to be polished, a so-called compression deformation is caused. Therefore, if pressing the semi-conductor wafer 5 to the polishing cloth 2 of the original state as shown in FIG. 13B, the semi-conductor wafer 5 sinks deeply in the polishing cloth 2 (the polishing cloth 2 is elastically deformed).
If the semi-conductor wafer 5 is polished under this condition, a down-part B of the polishing cloth 2 is large, and the circumferential part of the semi-conductor wafer 5 contacting the down-part B more advances the polishing than other parts, causing the so-called sags.
On the other hand, as seen in FIG. 13A, when the polishing cloth 2 is already elastically deformed and prior to recovering to the original state, the semi-conductor wafer sinks shallowly in the polishing cloth 2, and as the down-part B of the polishing cloth 2 is small, the sags are suppressed. Thus, based on this finding, the present invention has been established.
For accomplishing the above mentioned objects, the polishing apparatus of the invention is equipped with a polishing level block being rotatable and furnished with a polishing cloth for polishing semi-conductor wafers, a rotatable polishing head disposed in opposition to the polishing level block, and a polishing plate provided to the polishing head, said polishing plate being attached with a plurality of semi-conductor wafers, and a polishing operation being carried out as pressing said polishing cloth to said semi-conductor wafers, and is characterized by providing a guide ring disposed at the outside of the polishing plate for pressing the polishing cloth independently of polishing plate, a retainer ring provided at the lower end part of the guide ring for contacting to the polishing cloth, and a weight detachably mounted on the upper surface of the guide ring for adjusting pressure exerting on the polishing cloth.
As the polishing apparatus according to the invention detachably mounts weights on the upper surface of the guide ring, pressure of the retainer ring is easily controlled and has no probability of varying. Even in a case of the retainer ring having a large diameter as a batch type polishing apparatus for simultaneously polishing the plural semi-conductor wafers, substantially uniform pressure can be effected all over the retainer ring. Further, the retainer ring is provided at the lower end of the guide ring for contacting the polishing cloth, and therefore before the deformation of the polishing cloth recovers to the original shape (under the condition where the polishing cloth is already deformed), the semi-conductor wafers can be polished in mirror-finishing at high precision, enabling to avoid sags without equipping the retainer ring as encircling the outer circumference of each wafer.
The retainer ring is furnished for contacting the polishing cloth, so that the excessive polishing liquid is interrupted. Thus, the polishing liquid is not much supplied to the circumference of the semi-conductor wafer, enabling to prevent sags.
Herein, it is desirable to arrange an inside retainer for pressing the polishing cloth at the inside of the polishing plate.
Since the inside retainer is arranged inside of the polishing plate, elastic recovery at the inside of the polishing cloth is avoided, and prior to recovering the deformation of the polishing cloth (under the condition where the polishing cloth is already deformed), the polishing of the semi-conductor wafer is carried out. Therefore, it is possible to prevent collision between the semi-conductor wafer to be polished and the polishing cloth, enabling to prevent sags.
Herein, it is desirable that the inside retainer is composed of any of SiC and Al2O3.
Since the inside retainer is composed of any of SiC and Al2O3, it is excellent in abrasion resistance, enabling to avoid contamination of the semiconductor wafer to be polished.
In addition, it is desirable that the inside retainer is biased toward the polishing cloth by an elastic member, preferably, the inside retainer is provided at a front end of the holder to be biased by the elastic member.
Since the inside retainer is biased toward the polishing cloth by an elastic member, the pressure to the polishing cloth can be easily effected by changing the elastic member.
The weight is composed of plural ring-shaped weights each of which has a predetermined weight, and the pressure of the guide rings is desirably adjusted by selecting the ring shaped weights and mounting on the upper surface of the guide ring.
By combining the plural ring-shaped weights having the predetermined weight, specific pressing weight can be provided, so that the pressure of the retainer can be controlled.
It is desirable that the pressure of the retainer ring to the polishing cloth is around two to four times of the pressure of the semi-conductor wafer to the polishing cloth.
With this structure, the polishing cloth is pressed by the retainer ring, causing a sufficient compression deformation. Therefore before the deformation of the polishing cloth recovers to the original shape (under the condition where the polishing cloth is already deformed), the semi-conductor wafers can be polished. That is, the semi-conductor wafer to be polished does not sink in the polishing cloth, and is mirror-finished at high precision without causing avoid sags at the outer circumference of the wafer.
If the pressure of the retainer ring to the polishing cloth is below two times of the pressure of the semi-conductor wafer to the polishing cloth, since the polishing cloth does not cause the sufficient compression deformation, the deformation of the polishing cloth recovers to the original state before polishing the semi-conductor wafer, and the semi-conductor wafer sinks in the polishing cloth, causing avoid sags at the outer circumference of the wafer.
If the pressure of the retainer ring to the polishing cloth is beyond around four times of the pressure of the semi-conductor wafer to the polishing cloth, the polishing cloth causes a so-called plastic deformation, and the polishing cloth is undesirably not even in the surface.
It is desirable that that the retainer is a ceramic material composed of any of SiC and Al2O3.
As that the retainer is composed of any of SiC and Al2O3, it is excellent in abrasion resistance, enabling to avoid contamination of the semi-conductor wafer to be polished.
It is desirable that the holder and/or the guide ring are formed with PVC.
Neither holder nor guide ring require the abrasion resistance because of not contacting the polishing cloth, and it is desirable to employ resin materials such as PVC having rigidity and relatively light weight.
Further, it is also an object of the invention to provide a wafer polishing method which can display the effect of the retainer ring to the most and prevent sags in the peripheral part of the wafer.
According to this aspect of invention, a wafer polishing method is performed in the steps of dropping a slurry on a level block furnished with a polishing cloth thereon, pressing and rotating a wafer holding plate for polishing the wafers on the surface thereof, characterized by attaching retainer rings on the plate such that clearances are formed at peripheries of the wafers, and polishing the wafers as automatically adjusting the clearances.
Still further, a second aspect of the invention is such that the automatic adjustment is performed by pasting the retainer rings on the wafer holding plate making use surface tension of water.